Ip expA Eext kT

The relationship between current and potential is called the "characteristic" of the device, and the relation (4.62) for the p-n junction is illustrated in Fig. 4.7 by the curve labelled "no light". The constant saturation current I(p) is sometimes referred to as the "dark current".

Solar cells

A p-n junction may be utilised to convert solar radiation energy into electric power. A solar cell is formed by shaping the junction in such a way that, for example, the p-type material can be reached by incident solar radiation, e.g. by placing a thin layer of p-type material on top of a piece of n-type semi conductor. In the dark and with no external voltage, the net current across the junction is zero, as was shown in the previous subsection, i.e. the intrinsic potential difference A0O is unable to perform external work.

Figure 4.7. Characteristics (i.e. current as a function of external voltage) of a p-n junction, in the dark and with applied light. The magnitude of the short-circuit current, Is, is a function of light intensity and spectral distribution.

However, when irradiated with light quanta of an energy E^t = hv = hc/X (h is Planck's constant, c is the velocity of light and v and X are the frequency and wavelength of radiation), which is larger than the energy difference between the conduction and valence band for the p-type material,